To bend or not to bend?
That is the question if you’re talking about bending reinforcing steel bars (rebar) that are partially embedded in concrete, as we do in the Monolithic Dome construction process.
Fortunately, in building a Monolithic Dome only a minimum of the rebar used generally requires bending. We bend the rebar embedded in the footing down to get the Airform over it. Later, we bend it up so that the rebar becomes included in the concrete shell. This connects the shell to the footing or ring beam.
But does that bending weaken or damage the rebar? Three studies by professional researchers strive to answer that very question. Here is a summary of their findings that particularly apply to our process, along with information on where you can read the entire article:
Article: “How Harmful is Cold Bending/Straightening of Reinforcing Bars?”
Authors: José I. Restrepo, Senior Lecturer in Civil Engineering at the University of Canterbury, New Zealand and Member of the ACI (American Concrete Institute); Francisco J. Crisafulli, Senior Lecturer at the National University of Cuyo, Mendoza, Argentina and ACI Member; Robert Park, Professor of Civil Engineering, Deputy Vice Chancellor at the University of Canterbury, New Zealand and ACI Honorary Member.
Publication: Concrete International, April 1999, pages 45-48
Summary: This study took place in New Zealand where concrete construction standards discourage bending and straightening rebar that’s partially embedded in concrete if the procedure is not pre-approved by the design engineer or supervisor. In America, our ACI 318-95 building code allows cold bending (bending rebar without first applying heat) and straightening if authorized in the design drawings or by the site engineer. Consequently, the researchers were anxious to learn the effects of this somewhat suspicious practice.
They conducted 56 tensile or stress tests, on rebar manufactured in New Zealand (grades 300 and 420) that compares favorably with American rebar in steel grades of 40 and 60. In a significant number of these tests, they cold bent 20.5 inch lengths (520 mm) of #3 (3/8" diameter) and #4 (1/2" diameter) rebar.
The New Zealanders found that cold bending and straightening of rebar embedded in concrete does not weaken the bars if they are not bent through an angle greater than 90 degrees and the procedure is used only once.
Article: “Field Bending and Straightening of Reinforcing Steel”
Authors: Khossrow Babael, Consultant specializing in reinforced concrete and ACI Member; Neil M. Hawkins, Department Head, Civil Engineering, University of Illinois, Urbana-Champaign and ACI Fellow.
Publication: Concrete International, January 1992, pages 67-72
Summary: These researchers began their study after first completing an “extensive literature survey.” Their concern stemmed from reports – infrequent but worrisome – of cracking when rebar was bent and straightened and from engineering concerns about the strength of bent/straightened rebar.
This carefully monitored study used Grade 60, #4 and #6 (3/4" diameter) rebar in five different cold bending tests, and Grade 60, #7 (7/8" diameter) and #9 (1 1/8" diameter) in seven different hot bending tests.
Researchers found, “None of the test specimens broke during the bending or straightening operations.” But some micro-cracking was discovered. “Thus, the larger the bar, the more susceptible it is to cracking caused by bending/straightening operations.”
Article: “Field bending of rebars partially embedded in concrete: What the code allows, and supplemental findings by researchers”
Publication: #C830244, Copyright © 1983, The Aberdeen Group
Although this article addresses the problem of rebar partially embedded in concrete that was fabricated or placed incorrectly and must therefore be cold or hot bent, its data is of interest to us. For example, the article quotes ACI 318-77, “Building Code Requirements for Reinforced Concrete Structures”:
7.3.1 – All reinforcement shall be bent cold, unless otherwise permitted by the Engineer.
7.3.2 – Reinforcement partially embedded in concrete shall not be field bent, except as shown on the design drawing or permitted by the Engineer.
The article also quotes ACI 318R-77, 7.3.2 which, in part, states, “…The inspecting engineer must determine whether the bars can be bent cold without damage, or if heating is necessary. If heating is required it must be controlled to avoid splitting of the concrete or damage to the bars.”
Other reports – some that agree and some that disagree with ACI 318R-77, 7.3.2 – are also cited:
William C. Black in “Field Corrections to Partially Embedded Reinforcing Bars,” ACI Journal, October 1973, pages 690-691 recommends pre-heating rebar before gently and gradually bending it.
But J.R. Lalik and R. L. Cusick in “Cold Straightening of Partially Embedded Reinforcing Bars,” Concrete International, July 1979, pages 26-30 disagree. They said that it was not necessary to use heat before bending the rebar. However, they only tested #8 (1" diameter) bars.
In a rebuttal to Lalik and Cusick, L. A. Erasmus in “Cold Straightening of Partially Embedded Reinforcing Bars – A Different View,” Concrete International, June 1981, pages 47-52 said that it was hazardous to re-straighten cold rebar.
Based on these and other studies, the Concrete Reinforcing Steel Institute concluded that: reworking bars entails some risk; #8 or smaller bars can be successfully field bent/straightened at temperatures above 32° F; rebars #9, #10 and #11 have a better chance of being successfully bent/straightened when preheated to 1400° or 1500° F and carefully manipulated; these conclusions do not apply to #14 or larger rebar.
After reviewing the three articles summarized here, as well as other studies, Monolithic’s President David B. South said, “All of these studies show that care must be taken, but the smaller rebar sizes #3, #4, #5 and #6 usually can be bent cold as needed. However, care should be taken to make smooth bends, not kinks. Generally, the rebar is sufficiently forgiving – especially #3 and #4. Larger bars (larger than #6) take special handling and may require heating.”